Currently, as post-treatment devices that treat an exhaust gas of an engine (internal combustion engine), there are included a DPF (diesel particulate filter) that collects PM (particulate matter), and catalyst devices, such as a deNOx catalyst (nitric oxide compound reduction catalyst), a DOC (diesel oxidation catalyst), and a urea SCR catalyst (urea selective catalytic reduction). When a post-treatment device is actually provided in an exhaust passage of the engine, the above-described devices are combined with each other to purify the exhaust gas (for example, refer to Patent Documents 1, 2, and 3).
In the exhaust gas being purified by this post-treatment device, it is important to raise an exhaust gas temperature to a high temperature of approximately 200 to 250° C., and to activate the catalyst of the catalyst device. In addition, in PM collected by the DPF being burned, and the DPF being regenerated, it is necessary to feed fuel, burn it, and to raise a temperature of the fuel.
Consequently, a post injection immediately after a main injection is performed by an injector so that the exhaust gas is made into a high temperature state, and unburned fuel is discharged from an exhaust valve.
When performed in a state where an exhaust gas temperature in a cylinder is decreased, this post injection causes a problem that fuel adheres to a cylinder liner to be diluted in oil, and that durability of the engine is deteriorated. In addition, there is also a problem that thermal decomposition of feed fuel does not proceed, either, and that decomposition to a hydrocarbon (HC) component favorable for the catalyst does not proceed, since the fuel is injected into the cylinder in a low-pressure and low-temperature state.
Although there is also a method to add the HC directly to an exhaust pipe for dealing with this problem, and fuel dilution in the oil can be prevented by this method, an injection is performed to the exhaust pipe with a lower-temperature and a lower-pressure than an inside of the cylinder, and thus a problem cannot be solved that the thermal decomposition of the fuel takes more time than the post injection.